Gas turbine engines conventionally include a rotary compressor, a turbine, and a rotary shaft interconnecting the two. As is typical in equipment of all sorts having rotatable components, except when exotic bearings, such as magnetic bearings, are employed, it is necessary to provide for lubrication of the rotary components. In typical gas turbine engines, lubricating oil is provided to bearings journalling the rotary components, recovered and then recycled. These systems require pumps for recovering the lubricating oil as well as for circulating the lubricating oil. While such systems perform quite adequately, they can be heavy and/or bulky, not to mention expensive in construction. As a consequence, they are not suitable for use in all gas turbine systems.
For example, cruise missiles and target drones used by the military are frequently powered by small turbojet engines. Because these airborne vehicles are intended to be used, in the case of a cruise missile, but a single time, and in the case of target drones, no more than a couple of times, the turbojet engines employed are designed to be inexpensive to thereby provide an expendable engine. It accordingly follows that it is desirable that engine supporting systems, including the lubrication system, likewise be inexpensive as well. And because such engines are frequently used in airborne vehicles, it is highly desirable to minimize weight so that payload and/or range may be maximized.
At the same time, the lubrication system must be capable of operating reliably for the life of the engine and over a wide range of temperatures, typically from minus 40° F. to plus 180° F. Because these engines typically operate at a high rpm, a shaft or gear driven pump system is impractical as well as expensive.
Typically, the engines employed are relatively small and consequently, the lubricant flow rate is similarly small. Nonetheless, the flow must be reliable and delivered within the desired range under any and all conditions of operation. Typically, oil flows in the range of 1.5 cc per minute to 2.5 cc per minute are employed. To reliably obtain such flows when the oil experiences substantial changes in viscosity, dependent upon ambient temperature, poses substantial difficulty. Too little oil flow results in bearing failure and too great of an oil flow can result in premature exhaustion of oil and bearing failure.
Specifically, the nature of the system is that the maximum rate for the total oil flow has to be limited to assure that lubricating oil is available at or near the end of the mission cycle. Furthermore, the maximum rate has to be limited so as to enable the minimization of the size of the oil tank. Moreover, the system additionally has to be capable of being stored in the state of non-use with its compliment of lubricating oil for up to 15 years without loss and at the same time be ready for use immediately upon demand.
The present invention is directed to providing a lubricating oil system meeting these and other needs.